2-line YC separation devices are increasingly drawing attention as inexpensive means for accurately separating luminance and chrominance (YC) signals.
FIG. 14 is a block diagram of a 2-line YC separation device disclosed in Japanese Laid-open Patent No. H1-117494.
In FIG. 14, the reference numeral 2201 is the input video signal, 2202 is a one horizontal scanning time delay circuit (1HDL), 2203 is a subtractor, 2204 is also a subtractor, 2205 is a band-pass filter circuit (BPF), 2206 is a switch circuit, 2207 is a color subcarrier frequency trap circuit (T), 2208 is a switch circuit, 2209 is a luminance signal output, 2210 is a chroma signal output, 2211 is a low-pass filter circuit (LPF), 2212 is a YL-correlation detector (YL), 2213 is a band-pass filter circuit (BPF), 2214 is a chroma detector (C), and 2215 is an AND circuit.
The operation of a 2-line YC separation circuit configured as above is described below.
First, a signal, after passing through the one horizontal scanning time delay circuit 2202, is subtracted from the current video signal. Then, this signal is made into a YL correlation output signal, through the low pass filter circuit 2211 and the YL correlation detector 2212, and a detection output signal, through the chroma detector 2214 after filtering signals around the center of the color subcarrier frequency with the band-pass filter 2213. Both signals pass through the AND circuit 2215 and the output of AND circuit 2215 controls a comb-line filter.
More specifically, the switch circuit 2208 is turned on and, at the same time, the switch circuit 2206 is switched to the current video signal 2201 side only when the YL-correlation detector 2212 determines that i) there is no correlation, and ii) the chroma detector 2214 detects a chroma signal.
In other cases, for example, if the YL correlation detector 2212 determines that there is a correlation, or if the YL correlation detector 2212 determines that there is no correlation but the chroma detector 2214 detects no chroma signal, the switch circuit 2208 is turned off and the switch circuit 2206 is switched to the band-pass filter circuit 2205.
With the above configuration, however, YC in the input signal, with patterns illustrated as signal a in FIGS. 15, 16, and 17, cannot be accurately separated. Signal a in FIG. 15 is a pattern of a vertical line on a screen in which a luminance signal in the vertical direction, whose frequency is equivalent to that of the color subcarrier, exists up to (n+3)H and the luminance signal disappears thereafter. Signal a in FIG. 16 is a pattern in which a chroma signal exists up to (n+3)H and the chroma signal level is reduced thereafter. Signal a in FIG. 17 is a pattern of a vertical line on a screen in which a luminance signal in the vertical direction, whose frequency is equivalent to that of the color subcarrier, exists up to (n+3)H, and the luminance signal level is reduced thereafter.
In FIGS. 15, 16, and 17, signal a in is the input video signal; signal b is the output signal of the one horizontal scanning time delay circuit 2202: Signal c is the output signal of the subtractor 2203, or (a-b); signal d is the output signal of the low-pass filter circuit 2211; Signal e is the output of the YL-correlation detector 2212; Signal f is the output of the chroma detector 2214; Signal g in is the chroma signal; signal h is the luminance signal. Note that the amplitude of signals c and g in FIG. 16, however, are illustrated in half.
Since the output signal of the band-pass filter circuit 2205 is the result of filtering a frequency component of the color subcarrier in the output signal of the subtractor 2203, the output signal of the subtractor 2203 and the bandpass filter circuit 2205 become identical when a pattern such as signal a in FIGS. 15, 16, and 17 is input. Signal e in FIGS. 15, 16, and 17 becomes "1" when there is YL correlation, and "0" when there is no YL correlation. Signal f in FIGS. 15, 16, and 17 is "1" when a chroma signal exists and "0" when there is no chroma signal. Signal i in FIGS. 15, 16, and 17 is the optimal (desirable, ideal) luminance signal output. Signal j in FIGS. 15, 16, and 17 is the optimal chroma signal to be output.
Looking at signals e and f in FIG. 15, there is no signal in (n+1)H, (n+2)H, (n+3)H, (n+4)H, (n+5)H, and (n+6)H when the YL-correlation detector 2212 does not detect correlation, and the chroma detector 2214 detects the presence of the chroma signal. In other words, there is no signal where signal e is "0", at the same time signal f is "1". Consequently, the switch circuit 2208 is turned off and the switch circuit 2206 is switched to band-pass filter circuit 2205. In this case, the luminance signal output h becomes the same as the result of subtracting the output signal of the band-pass filter circuit 2205, which is equal to signal c in FIG. 15, from the current video signal, which is signal a in FIG. 15. Therefore, signal h in FIG. 15, which is equal to (a-c), is output. The chroma signal output g is the output signal of the band-pass filter circuit 2205 which is equal to the output signal of the subtractor 2203, or signal c in FIG. 15, and therefore signal g in FIG. 15 is output.
In the same way, looking at signals e and f in FIG. 16, there is no signal in (n+1)H, (n+2)H, (n+3)H, (n+4)H, (n+5)H, and (n+6)H when the YL-correlation detector 2212 does not detect correlation, and the chroma detector 2214 detects the presence of the chroma signal In other words, there is no signal where signal e is "0" at the same time signal f is "1". Consequently, the switch circuit 2208 is turned off and the switch circuit 2206 is switched to band-pass filter circuit 2205. In this case, the luminance signal output h becomes the same as the result of subtracting the output signal of the band-pass filter circuit 2205, which is equal to the output signal of the subtractor 2203, or signal c in FIG. 16, from the current video signal, or signal a in FIG. 16. Therefore, signal h in FIG. 16, which is equal to (a-c), is output. The chroma signal output g is the output signal of the band-pass filter circuit 2205 which is equal to the output signal of the subtractor 2203, or signal c in FIG. 16, and therefore signal g in FIG. 16 is output.
In the same way, looking at signals e and f in FIG. 17, there is no signal in (n+1)H, (n+2)H, (n+3)H, (n+4)H, (n+5)H, and (n+6)H when the YL-correlation detector 2212 does not detect correlation, and the chroma detector 2214 detects the presence of the chroma signal. In other words, there is no signal where signal e is "0" at the same time signal f is "1". Consequently, the switch circuit 2208 is turned off and the switch circuit 2206 is switched to the band-pass filter circuit 2205. In this case, the luminance signal output h is the same as subtracting the output signal of the band-pass filter circuit 2205, which is equal to the output signal of the subtractor 2203, or signal c in FIG. 17, from the current video signal, which is signal a in FIG. 17. Therefore, signal h in FIG. 17, which is equal to (a-c), is output. The chroma signal output g is the output signal of the band-pass filter circuit 2205 which is equal to the output signal of the subtractor 2203, or signal c in FIG. 17, and therefore signal g in FIG. 17 is output.
This means that in a pattern such as signal a in FIG. 15, comprising the luminance signal in a vertical direction on a screen, whose frequency is equivalent to that of a color subcarrier, which exists up to (n+3)H, and disappears from (n+4)H, the luminance signal displayed will be signal h in FIG. 15, and the chroma signal displayed will be signal g in FIG. 15. In a pattern such as signal a in FIG. 16 comprising the chroma signal up to (n+3)H with its level reduced from (n+4)H, the luminance signal displayed will be signal h in FIG. 16 and the chroma signal displayed will be signal g in FIG. 16. In a pattern such as signal a in FIG. 17 comprising the luminance signal up to (n+3)H with its level reduced from (n+4)H, the luminance signal displayed will be signal h in FIG. 17 and the chroma signal displayed will be signal g in FIG. 17. On the other hand, the optimal luminance signal to be output is signal i and the optimal chroma signal to be output is signal j in FIG. 15 when the input is signal a in FIG. 15. When signal a in FIG. 16 is input, the optimal luminance signal is signal i and the optimal chroma signal is signal j in FIG. 16. When signal a in FIG. 17 is input, the optimal luminance signal is signal i and the optimal chroma signal is signal j in FIG. 17.
As described above, conventional technology is incapable of separating YC signals correctly, and may result in erroneous display operation, in such as degraded horizontal resolution, appearance of color where there should be no color, and appearance of luminance components where color should be.